For the first time in their respective political careers, US Senators Harry Reid (D-NV) and John McCain (R-AZ) both face uphill battles for re-election this fall. But the issue most likely to decide each man’s fate isn’t the state of the economy, health care, or climate change. It’s immigration reform, a cause that each has championed in the past but which now threatens their respective candidacies in starkly different ways.

McCain, a four-term senator who typically wins in a landslide, faces his stiffest challenge ever from Tea Party favorite J.D. Hayworth, who has surged in the polls by attacking McCain’s support for an “amnesty” for illegal immigrants, while touting his own plan to institute a general “moratorium” on immigration – both legal and illegal – to slow down what Hayworth’s depicts as the creeping “Mexicanization” of the American Southwest.

Since 2006, Arizona has passed some of the most draconian anti-immigration legislation anywhere in the country, including a harsh employer sanctions law, and more recently, legislation encouraging Arizona police to identify and arrest illegal aliens. In response, McCain has endorsed a plan to position 3,000 National Guard troops along the Arizona-Mexico border, effectively conceding the immigration issue to Hayworth. And a month ago, when President Obama met with congressional leaders to discuss whether comprehensive immigration reform could be passed in 2010, McCain made a point of speaking out, saying “now is not the time.” McCain has even endorsed the state’s new police enforcement law, despite widespread concerns about the measure’s legality.

Reid, meanwhile, faces almost the opposite problem from McCain. He trails any one of his likely GOP challengers by 8-10 points in the polls, and his electoral prospects, by all accounts, are dimming. But unlike Arizona, the immigration mood in Nevada is far more “pro” than “anti.” Nevada’s not a border state, and much of its rapidly expanding Latino population – now 15% of the electorate – has migrated from California to service Arizona’s burgeoning gambling and tourist industry. These are badly needed workers, and their presence is largely welcomed.

Moreover, the state’s teachers unions are dominated by Latina women, who constitute an unusually potent Democratic grassroots campaign network. Thanks largely to union politicking, more than three-quarters of Nevada’s Hispanics ended up voting for Obama in 2008, compared to just over half in Arizona. Nevada’s Hispanics are also staunchly in favor of comprehensive immigration reform, and like Hispanics elsewhere, have grown critical of Obama – and increasingly, Reid – for failing to move the issue forward, as promised.

Which is precisely why Reid’s now considering a big new push on immigration reform. The four-term senator and current majority leader first announced the new push in mid-April at a campaign rally in Las Vegas. He has since spoken with House Speaker Nancy Pelosi (D-CA), the two have apparently agreed to tackle the issue, once the current debate over financial reform is settled. President Obama, by all accounts, is also on board, and has recently called upon Congress to step up to the plate, or risk “more irresponsible actions” – a thinly-veiled reference to Arizona.

But some observers – and not just Republicans – are skeptical of Reid’s “Hail Mary” on immigration. For one thing, the two main Senate architects of reform, Sen. Charles Schumer (D-NY), and his GOP counterpart, Sen. Lindsay Graham (R-SC), are not yet on board – at least not publicly. Graham, in fact, is on record as saying that he would not support immigration reform’s passage in 2010, in part because the Democrats insisted on passing health care reform through budget reconciliation, and refused to accede to GOP demands for further debate. Graham’s the leading Republican moderate on immigration, and without his active support, it’s hard to imagine Democrats rounding up enough GOP cross-over votes to offset opposition to reform from within their own party.

Which leaves the Democrats in the unenviable position of trying to forge their own narrow consensus on immigration reform, possibly just to embarrass the GOP in an election year. Democrats could try to have the Senate vote on a bill, without necessarily seeking to fight for the bill to be passed by both houses. That prospect frightens Tamara Jacoby, executive director of Immigration Works, who warned in a recent Washington Post op-ed that immigration advocates have only one chance before 2012 to pass a serious immigration reform bill. Pushing the issue too soon, for strictly political reasons, she maintains, could “doom the reform cause for years.”

The fact is Congress is still dead-locked on reform. The broad outlines of a reform bill are well-known: supporters want a sweeping legalization program for undocumented workers while opponents are demanding stepped up border and workplace enforcement, including the establishment of a national ID card. But even if this broad trade-off could be agreed to, several perennially contentious issues remain, including whether a temporary “guest worker” program should be established – McCain, Graham and business groups strongly support the idea over the objections of Obama’s labor allies – and whether high-skilled workers should be privileged over low-skilled ones in future visa admissions, which is anathema to most advocacy groups.

There’s even a new source of controversy that threatens to tear the entire immigration reform coalition apart: whether the right of US citizens and legal immigrants to sponsor their spouses for green cards should be extended to gay and lesbian couples who are not technically married. Most reform advocates, including many religious groups, support the provision, known as the “Uniting American Families Act,” or UAFA, as it simply recognizes the reality of same-sex unions across borders, and as currently written, could also apply to heterosexual common-law couples. But the US Catholic Church and leading Christian evangelical organizations see it as a flat-out endorsement of gay marriage and are threatening to oppose the immigration reform bill if the UAFA provisions are included.

And, of course, unemployment still stands at 10%, an issue that GOP opponents of immigration will exploit to the hilt. That, plus the continuing intra- and inter-party divisions, does not bode well for the passage of a comprehensive reform bill this year.

But there’s an alternative to doing nothing on immigration or doing way too much – as Reid is contemplating – but without sufficient support. That’s to do just a little, and to focus on the least contentious aspects of reform. For example, some of the partial legalization schemes – such as the DREAM Act and the Ag Jobs Bill – could be sandwiched together with an expansion of the current workplace verification system known as “E-Verify.” Additionally, Congress could vote clear up the appalling backlog in visa processing for legal immigrants – a move even Sarah Plain endorses – and also rectify the dismal state of immigrant detention facilities – again, a relative no-brainer. And throw in some added funding for border enforcement. Then call it a day.

The advantage of this approach? Each side gets to claim partial victory – advocates because they succeed in legalizing several million undocumented workers, and opponents because they derailed, at least for now, a full amnesty, while securing new resources for enforcement. And while the real sticking points in the debate are postponed until the next legislative session, Republicans and Democrats could each go into November claiming to have supported immigration reform. In other words, a partisan draw, but a victory for the issue.

Officially, Schumer says he opposes a “piecemeal” approach to immigration reform. But Schumer’s been known to say many things officially – not all of them true, or even viable – only to change his mind when the political winds shift. And while most of the leading immigrant advocacy groups prefer to see all undocumented immigrants legalized en masse, they are growing anxious about a GOP takeover in November that could push this maximal goal further out of reach. Graham might well support the maneuver, as would many Republicans, including, quite possibly, McCain.

Even without McCain’s support, immigration reform of some sort is now clearly at play in the 2010 election. But for Reid’s gambit to succeed, advocates may have to consider downsizing their legislative ambitions, or risk a final, catastrophic collapse. And it wouldn’t hurt to fashion a bill that might defuse the issue in Arizona, ensuring McCain’s re-election, while denying an anti-immigrant firebrand like Hayworth a seat in the new Senate.